The invention relates to a fuse device, in which a thick film fusible conductor is applied to an upper surface of an electrically insulating substrate, and to a method of manufacturing such a fuse device.
Fuse devices of the type referred to above are disclosed in the prior art in a series of publications. Reference is made by way of example to the fuse for SMD installation described in WO 96/41359 A1. Formed on a rectangular surface of an insulating substrate, which consists, for instance, of Al2O3, between two connecting surfaces, is a metallic thick film fusible conductor. The connecting surfaces are formed on opposing edges of the surface of the substrate and are composed of a plurality of metal layers and are provided for the purpose of SMD installation with a solderable coating. A spot comprising a layer, which contains tin/lead, is applied to a central section of the fusible conductor applied to the surface of the substrate. The configuration is so designed that in the event of predetermined current flows of predetermined minimum durations the fusible conductor and the spot applied on it heat up to an extent which is sufficient to soften or to melt the material of the spot to the extent that the tin/lead metal diffuses into the metal of the fusible conductor disposed beneath it. This locally increases its electrical resistance, which results in an increased voltage drop, an increased local power loss, further heating and finally in melting and/or vaporisation of the material of the fusible conductor. The current which results in the described manner in rupturing of the fusible conductor is less than the current which would be necessary for melting the fusible conductor without the applied tin/lead spot. However, as a result of the described, time-consuming processes, a considerably longer time of the current flow is necessary until rupture (tripping); the fuse device is very “sluggish”.
On the other hand, U.S. Pat. No. 5,166,656 discloses a very rapidly acting SMD fuse for protecting electronic circuits, in which a metallic thin film fusible conductor with a thickness of 0.6 to 4.5 μm is applied to a glass substrate and is covered with a passivation layer of CVD SiO2 or imprinted glass, whereafter a second glass plate is secured to it with an adhesive layer (epoxide).
Slow acting fuses of small size are required, for instance, in telecommunication devices, particularly to protect input circuits or interface circuits, which are coupled to long transmission lines. These transmission lines are subjected to the influences of electric and magnetic fields which are produced by lightening strikes and high voltage cables extending in the vicinity. These influences can result, amongst other things, in brief current/voltage pulses with high peak values on the telecommunication signal transmission lines, which can potentially damage the devices connected to them, particularly their input circuits. The input connections of the device are thus protected against over-voltages and, with the aid of fusible protection devices, against excessive currents. These telecommunication devices or their fuse devices are subjected to complicated requirements, which are specified in a series of special tests. On the one hand, “telecommunications” fuse devices should reliably trip (that is to say no longer enable the flow of current even by way of an arc) at currents of predetermined magnitude within predetermined maximum current flow periods (e.g. at 40 A within 1.5 s or at 7 A within 5 s). Furthermore, the fuse devices should be slow acting, that is to say if their maximum permissible current is slightly exceeded they trip (rupture) after a relatively long duration of the current flow. Finally, they should be able to resist brief (in the millisecond range) relatively large currents of up to 100 A without tripping (such currents are produced e.g. in the event of over-voltage pulses, which are dissipated to earth by an over-voltage protective device with a low internal resistance, whereby the current which is produced flows via the fuse element). The requirements on devices with “telecommunications” fuse devices are specified e.g. in the “UL 1950”, “FCC Part 68” and “Bellcore 1089” tests.
It is the object of the invention to provide a fuse device which renders it possible to satisfy the requirements referred to above with a small structural size and low manufacturing costs and which furthermore can be constructed in the form of an SMD component.